Multimode fiber may be used, for example, for super computer interconnects, within data centers, local area networks, or other applications requiring relatively short point-to-point link lengths. Examples of multimode fiber include but are not limited to 62.5 μm and 50 μm core-diameter multimode optical fibers, step index multimode fibers, graded index multimode fibers (examples of which include but are not limited to OM1 and OM2/3/4), and optical fibers that support only two, three or more modes.
Generally, in use many or all of the multimode fiber's plurality of spatial modes are excited with the same optical signal. Symbols within the optical signal are increasingly dispersed with propagation distance because some of the plurality of spatial modes have different group delay. This results in inter symbol interference that may limit transmission to a few hundred meters, for example.
The plurality of spatial modes that a multimode optical fiber supports may depend on the geometry, index contrast, and other parameters. A multimode fiber that only supports no more than a few spatial modes is commonly known as a “few moded fiber”. Example conventional labels for a selection of spatial modes that may be found in a multimode fiber include: LP01 (the fundamental mode), LP11, LP21, LP02, LP31, LP12, LP41, LP22, LP03, LP51, LP32, LP61, LP13, and LP42 (higher order modes), and degeneracies thereof, for example LP11a and LP11b. FIGS. 1 and 2 shows graphs of normalized propagation constants versus the fiber parameter for each of the above mentioned modes.
Most optical signal transmission systems of relatively longer length use an optical fiber that supports a single spatial mode to eliminate the above mentioned source of inter symbol interference.
It has been appreciated, however, that the information carrying capacity of an optical signal transmission link may be increased by using multimode optical fiber in a non-traditional manner. The plurality of spatial modes of some graded index multimode optical fiber may be arranged into spatial mode groups. In one definition of spatial mode groups, each spatial mode group may comprise spatial modes with approximately the same phase constant and group delay. Consequently, modes within a mode group may couple easily. The coupling between two modes of two groups, however, may be significantly less. Spatial mode groups may be defined in other ways. Over some distance, it may be possible to consider all mode members of a mode group as part of the same channel to form a spatial mode group division multiplexed system. Inter symbol interference within each channel may be significantly less than traditionally experienced in multimode fiber links, which may be exploited for increasing the data rate per channel or increasing the link length.
In a circularly symmetric multimode optical fiber, linear polarisation modes LPlm are determined by their azimuthal order l and their radial order m. At least some modes with the mode number n satisfying the equation n=l+2m+1 may have approximately the same phase constant and group delay and may belong to a mode group with order n.
For example, mode group 1 generally may comprise the LP01 spatial mode, which is the fundamental (zero order) mode. The second mode group may generally comprise LP11a and LP11b spatial modes. Some but not all multimode fibers support a third mode group comprising LP02, LP21a and LP21b spatial modes. Some multimode optical fibers comprise other mode groups that comprise further higher order modes. Multimode optical fibers with a diameter of 50 μm, for example, may have approximately 9-10 mode groups.
Practical means to individually excite the plurality of spatial mode groups of the multimode fiber, and optically isolate each of the plurality of spatial mode groups are required.